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FE theory is refined by introducing the interaction between π electrons and an elastic σ skeleton. It is shown that even for very long polyenes a difference in single and double bond length of the same order of magnitude as in shorter polyenes is to be expected. Bond lengths in the ground and first excited state are calculated, which compare favorably with experimental data deduced by a Franck-Condon analysis of the intensities of vibrational peaks.
Heinrich Labhart (1957) studied this question.